In the dynamic balancing of a rotating part such as an electric motor armature, the part is mounted on its axis between bearings, rotated, and the unbalance is sensed by vibration or force sensors at the bearing locations. Several methods and devices have been developed to indicate the location of the unbalance on a rotated part. Two early types of machines widely used in industry use stroboscopic and photocell techniques to locate the unbalance. These both had the disadvantage of requiring physical markings on the part being rotated. These machines also required visual estimates of the unbalance location and were therefore subject to operator error.
The most advanced machine of this type is disclosed in U.S. Pat. No. 4,419,894 to Matumoto, wherein an unmarked workpiece is rotated, the unbalance measured and located, and the workpiece stopped with the unbalance position in a predetermined orientation for subsequent marking and material mass addition or removal. This machine utilizes vibration sensors to generate an analog unbalance signal which is sinusoidal. An unbalance phase pulse is then electronically generated once per cycle at the positive going zerocrossing of the unbalance signal. The workpiece is driven by a stepper motor. Each drive pulse supplied to the stepper motor causes the workpiece to rotate an unknown but fixed angle. A counter, preset with a number representing an integral number of stepper motor drive pulses, is counted back upon each stepper motor drive pulse, starting with the receipt of an unbalance phase pulse and the rotated workpiece is stopped when the counter reaches zero. It is a real time system in that pulses coming from the unbalance sensor are used to initiate the countdown.
There are several limitations and drawbacks associated with this type of machine. First, considerable time is required to initially set up the machine to maximize plane separation, select optimum counter settings, and set acceleration and deceleration rates to minimize belt slippage. These adjustments must be made for each different workpiece type measured. Settings are determined by trial and error methods which are awkward and time consuming.
Second, the Matumoto method does not verify the accuracy of the determination of the rotational speed and therefore introduces error due to inherent drive belt slippage between the stepper motor drive and the driven part.
Third, minor differences in armature diameters may introduce errors in unbalance positioning because the Matumoto machine does not measure and utilize the actual rotational frequency of the workpiece.
Finally, because the Matumoto method involves time consuming setup steps and inherent errors for each workpiece, it entails significant restrictions in efficiency for production line processing.